You can use the following media query to make image stack on mobile:

@media (max-width: 800px) {
    .wp-block-gallery .blocks-gallery-grid li.blocks-gallery-item {
        width: 100%;
    }
}

We have some plans to improve the mobile controls in the future but I’m not sure a stack on mobile option for the gallery is something that is planned at the moment.

I hope this information is useful, let us know if you have any further questions!

.wp-block-gallery .blocks-gallery-item {
    margin: 0 0 1em 0;
    width: 100%;
    }

    
@media only screen and (max-width: 992px) {
   li.blocks-gallery-item {
      width: 40%;
      margin: 1em;
   }
}